U.S. patent number 5,661,449 [Application Number 08/520,723] was granted by the patent office on 1997-08-26 for magnetic multilayer film, method for making, and magnetoresistance device.
This patent grant is currently assigned to TDK Corporation. Invention is credited to Satoru Araki, Daisuke Miyauchi.
United States Patent |
5,661,449 |
Araki , et al. |
August 26, 1997 |
**Please see images for:
( Certificate of Correction ) ** |
Magnetic multilayer film, method for making, and magnetoresistance
device
Abstract
A magnetic multilayer film having magnetoresistance (MR) is
prepared by alternately depositing first and second magnetic layers
while interposing a non-magnetic metal layer therebetween. The
number of the first magnetic layers N1 and the number of the second
magnetic layers N2 having greater coercivity than the first
magnetic layers are in the range: 2.ltoreq.N1.ltoreq.4 and N2=N1-1.
The first magnetic layer has a thickness t1 of 10-80 .ANG., second
magnetic layer has a thickness t2 of 20-90 .ANG., and non-magnetic
metal layer has a thickness t3 of 20-90 .ANG.. The magnetic
multilayer film comprising a less number of layers has a great MR
ratio, a linear rise of an MR curve in proximity to zero magnetic
field, and high sensitivity to a magnetic field. It also has a
greater MR slope upon application of a high frequency magnetic
field in proximity to zero magnetic field and withstands high
temperatures. The magnetic multilayer film is used as a
magneto-sensitive section to provide high performance MR
devices.
Inventors: |
Araki; Satoru (Chiba,
JP), Miyauchi; Daisuke (Nagano, JP) |
Assignee: |
TDK Corporation (Tokyo,
JP)
|
Family
ID: |
16859385 |
Appl.
No.: |
08/520,723 |
Filed: |
August 29, 1995 |
Foreign Application Priority Data
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Aug 29, 1994 [JP] |
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6-227347 |
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Current U.S.
Class: |
338/32R; 360/324;
257/E43.005; 324/252; 324/207.21 |
Current CPC
Class: |
H01F
10/324 (20130101); H01L 43/10 (20130101); B82Y
25/00 (20130101) |
Current International
Class: |
H01F
10/32 (20060101); H01F 10/00 (20060101); H01L
43/10 (20060101); H01L 43/00 (20060101); H01L
043/00 () |
Field of
Search: |
;338/32R ;360/113
;324/252,207.21 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 483 373 A1 |
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May 1992 |
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EP |
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4-218982 |
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Aug 1992 |
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JP |
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6-122963 |
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May 1994 |
|
JP |
|
Other References
Magnetoresistance of Multilayers with Two Magnetic Components, H.
Yamamoto, et al., Journal of Magnetism and Magnetic Materials, vol.
99, pp. 243-252, 1991. .
Large Magnetoresistance of Field-Induced Giant Ferrimagnetic
Multilayers, T. Shinjo, et al., Journal of the Physical Society of
Japan, vol. 59, No. 9, Sep. 1990, pp. 3061-3064. .
M.N. Baibich et al, Giant Magnetoresistance of (001)fe/(001)Cr
Magnetic Superlattices, vol. 61, No. 21, (1988), pp.
2472-2474..
|
Primary Examiner: Walberg; Teresa J.
Assistant Examiner: Easthom; Karl
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Claims
We claim:
1. A magnetic multilayer film comprising a first magnetic layer and
a second magnetic layer having a greater coercive force than the
first magnetic layer, said first and second magnetic layers being
alternately stacked with a non-magnetic metal layer intervening
therebetween, wherein
the number of the first magnetic layers N1 and the number of the
second magnetic layers N2 fall in the range:
the first magnetic layer has a thickness t1, the second magnetic
layer has a thickness t2, and the non-magnetic metal layer has a
thickness t3 wherein
2. The magnetic multilayer film of claim 1 wherein each said first
magnetic layer has a magnetization M1 and each said second magnetic
layer has a magnetization M2 wherein
3. The magnetic multilayer film of claim 1 wherein said first
magnetic layer has an anisotropic magnetic field Hk of 3 to 20
Oe.
4. The magnetic multilayer film of claim 1 wherein said first
magnetic layer comprises a magnetic metal containing at least 70%
by weight of a composition of the formula:
wherein x and y representative of weight ratios are
0.7.ltoreq.x.ltoreq.0.9 and 0.ltoreq.y.ltoreq.0.3,
said second magnetic layer comprises a magnetic metal containing at
least 70% by weight of a composition of the formula:
wherein z and w representative of weight ratios are
0.4.ltoreq.z.ltoreq.1.0 and 0.5.ltoreq.w.ltoreq.1.0, and
said non-magnetic metal layer comprises a non-magnetic metal
containing at least one element selected from the group consisting
of gold, silver, and copper.
5. The magnetic multilayer film of claim 1 wherein
said first magnetic layer comprises a magnetic metal containing at
least 70% by weight of a composition of the formula:
wherein x and y representative of weight ratios are
0.7.ltoreq.x.ltoreq.0.9 and 0.ltoreq.y.ltoreq.0.3,
said second magnetic layer comprises a magnetic metal containing at
least 70% by weight of at least one element selected from the group
consisting of cobalt, iron, and nickel and having added thereto at
least one element selected from the group consisting of Pr, Pt, Tb,
Gd, Dy, Sm, Nd, Eu, and P, and
said non-magnetic metal layer comprises a non-magnetic metal
containing at least one element selected from the group consisting
of gold, silver, and copper.
6. The magnetic multilayer film of claim 1 which upon application
of a DC magnetic field, provides a magnetoresistance curve having a
maximum slope of at least 0.2%/Oe in a magnetic field in the range
between -10 Oe and +10 Oe.
7. The magnetic multilayer film of claim 1 which upon application
of an AC magnetic field at a frequency of 1 MHz and an amplitude of
10 Oe over the range between -20 Oe and 20 Oe, provides a
magnetoresistance curve having a maximum slope of at least
0.1%/Oe.
8. A method for preparing a magnetic multilayer film as set forth
in claim 1 comprising the steps of forming the first magnetic
layer, the second magnetic layer and the non-magnetic metal
layer,
wherein during the step of forming the first magnetic layer, a
magnetic field is applied in one in-plane direction of the first
magnetic layer.
9. The method of claim 8 wherein the steps of forming the first
magnetic layer, the second magnetic layer and the non-magnetic
metal layer are by evaporation, particles being deposited
possessing an energy of 0.01 to 10 eV.
10. The method of claim 8 wherein the steps of forming the first
magnetic layer, the second magnetic layer and the non-magnetic
metal layer are by evaporation in an atmosphere of up to 10.sup.-8
Torr.
11. The method of claim 8 which further includes the step of heat
treating the resultant magnetic multilayer film at a temperature of
up to 300.degree. C.
12. The method of claim 8 which further includes the step of heat
treating the resultant magnetic multilayer film at a temperature of
up to 400.degree. C. in an atmosphere of up to 10.sup.-7 Torr.
13. A magnetoresistance device comprising a magnetic multi-layer
film as set forth in claim 1 as a magneto-sensitive section.
14. The magnetoresistance device of claim 13 which is free of a
bias magnetic field applying mechanism.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a magnetoresistance device capable of
reading a small magnetic field strength change as a greater
electrical resistance change signal, a magnetic multilayer film
suitable for use therein, and a method for preparing the magnetic
multilayer film. The term "magneto-resistance" is often abbreviated
as MR, hereinafter.
2. Prior Art
There are growing demands for increased sensitivity of magnetic
sensors and increased density of magnetic recording. Researchers
strive for the development of magnetoresistance effect type
magnetic sensors (simply referred to as MR sensors, hereinafter)
and magneto-resistance effect type magnetic heads (simply referred
to as MR heads, hereinafter). Both MR sensors and MR heads are MR
devices for measuring the strength of a magnetic field applied
thereacross by utilizing the principle that a reading sensor
portion of MR material changes its electric resistance in response
to an external magnetic field. The MR material detects the strength
of an external magnetic field itself. Unlike inductive magnetic
heads, the MR heads produce outputs which do not depend on their
speed relative to magnetic recording media, ensuring high outputs
upon reading of high density magnetic recording signals. The MR
sensors have the advantage of high sensitivity.
Conventional MR heads using magnetic materials such as Ni.sub.0.8
Fe.sub.0.2 (Permalloy) and NiCo as MR material have less enough
sensitivity to read ultrahigh density record of the order of
several GBPI since their percent MR ratios are low.
Attention is now paid to artificial superlattices having the
structure in which thin films of metal having a thickness of an
atomic diameter order are periodically stacked since their behavior
is different from bulk metal. One of such artificial superlattices
is a magnetic multilayer film having ferromagnetic metal thin films
and non-magnetic metal thin films alternately deposited on a
substrate. Heretofore known are magnetic multilayer films of
iron-chromium and cobalt-copper types. Among them, the
iron-chromium (Fe/Cr) type was reported to exhibit a MR ratio in
excess of 40% at cryogenic temperature (4.2K) (see Phys. Rev.
Lett., Vol. 61, page 2472, 1988). This artificial superlattice
magnetic multilayer film, however, is not commercially applicable
as such because the external magnetic field at which the MR ratio
becomes maximum (that is, operating magnetic field strength) is as
high as ten to several tens of kilooersted (kOe). Additionally,
there have been proposed artificial superlattice magnetic
multilayer films of Co/Ag, which require too high operating
magnetic field strength.
Under these circumstances, a ternary artificial superlattice
magnetic multilayer film having two types of magnetic layers having
different coercive forces deposited with a non-magnetic layer
interposed therebetween was proposed as exhibiting a giant MR
change due to induced ferrimagnetism. Regarding such magnetic
multilayer films, the following articles and patents are known.
(a) T. Shinto and H. Yamamoto, Journal of the Physical Society of
Japan, Vol. 59 (1990), page 3061.
Described is a magnetic multilayer film of
[Cu(x)--Co(y)--Cu(x)--NiFe(z)]xN wherein x, y and z represent the
thickness in angstrom of the associated layer and N is the number
of recurring units of Cu--Co--Cu--NiFe (the same applies
hereinafter) wherein (x, y, z, N)=(50, 30, 30, 15). It produced an
MR ratio of 9.9% at a maximum applied magnetic field of 3 kOe and
about 8.5% at 500 Oe.
(b) H. Yamamoto, Y. Okuyama, H. Dohnomae and T. Shinjo, Journal of
Magnetism and Magnetic Materials, Vol. 99 (1991), page 243
In addition to (a), this article discusses the results of
structural analysis, changes with temperature of MR ratio and
resistivity, changes with the angle of external magnetic field, a
minor loop of MR curve, dependency on stacking number, dependency
on Cu layer thickness, and changes of magnetization curve.
(c) U.S. Pat. No. 4,949,039 or JP-A 61572/1990
Ferromagnetic layers stacked alternately with non-magnetic
intermediate layers align anti-parallel, exhibiting great
magnetoresistance effect. A structure wherein an antiferromagnetic
material is disposed adjacent one of the ferromagnetic layers is
also disclosed.
(d) U.S. Pat. No. 5,315,282, EP 0 483 373 A1 or JP-A
218982/1992
Disclosed is a magnetic multilayer film having two types of
magnetic layers having different coercive forces stacked through an
intervening non-magnetic layer. An exemplary structure includes a
Ni-Fe layer of 25 .ANG. or 30 .ANG. thick, an intervening Cu layer,
and a Co layer of 25 .ANG. or 30 .ANG. thick.
(e) JP-A 122963/1994
Disclosed is a magnetic multilayer film having two types of
magnetic layers having different coercive forces stacked through an
intervening non-magnetic layer. By controlling the squareness ratio
of the two magnetic layers, the slope of an MR curve at zero
magnetic field is increased and the MR effect at high frequency is
improved.
As compared with Fe/Cr, Co/Cu and Co/Ag, these field-induced
ferrimagnetic multilayer films are inferior in the magnitude of MR
ratio, but experience a rapid change of MR ratio under an applied
magnetic field of less than several tens of oersted. They are thus
effective MR head materials coping with a recording density of
about 1 to 100 Gbits per square inch. While MR heads are required
to operate under a magnetic field at a high frequency of at least 1
MHz for high density writing/reading, the magnetic multilayer film
described in (e) is fully practical due to its improved MR effect
at high frequency.
For increased sensitivity, the MR head has the so-called shielded
structure wherein a magnetic multilayer film or Permalloy serving
as a magneto-sensitive section is interposed between a pair of soft
magnetic layers with an non-magnetic layer interposed therebetween.
The distance between the pair of soft magnetic layers, known as a
shield length, is very important. The shield length must be reduced
as the recording density increases. However, in conventional MR
heads using Permalloy in the magneto-sensitive section, the
magneto-sensitive section has an increased total thickness because
a shunt layer and a soft film bias layer are added to the
Permalloy. This prevents the shield length from being reduced,
leaving a problem. In MR heads using a magnetic multilayer film
having a great slope of a MR curve at zero magnetic field as
described in (e), the shield length cannot be reduced if there is a
large number of recurring units.
The shield length may be reduced by reducing the number of
recurring units N in a multilayer structure. In the above-referred
article (b), Journal of Magnetism and Magnetic Materials, Vol. 99
(1991), pp. 243-252, FIG. 9 is a diagram of MR ratio at RT as a
function of stacking number in a multilayer, which contains some
examples wherein the number of recurring units N is 3 or less. In
these examples, however, the percent MR ratio is as low as 4% at
N=3 and 1.2% at N=1. As seen from these examples, prior art
field-induced ferrimagnetic multilayer films have the problem that
if the number of recurring units N is reduced in order to reduce
the shield length, the percent MR ratio is concomitantly reduced.
Even with the magnetic multilayer films described in (e), where the
number of recurring units N is small, it is difficult to provide
satisfactory MR characteristics and the slope of a MR curve is
reduced especially under a high frequency magnetic field.
Since a complex laminate structure is used in MR heads or the like,
patterning and flattening steps require heat treatment such as
baking and curing of resist material, which in turn, requires heat
resistance at temperatures of about 300.degree. C. However, prior
art artificial superlattice magnetic multilayer films tend to
deteriorate by such heat treatment, and lose heat resistance
especially when the number of recurring units N is small.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magnetic
multilayer film which includes a smaller number of stacked magnetic
layers, and exhibits a greater percent MR ratio, a linear rise of
MR ratio in proximity to zero magnetic field, for example, under an
applied magnetic field between -10 Oe and +10 Oe, a high magnetic
field sensitivity, a greater slope of a MR curve under a high
frequency magnetic field applied in proximity to zero magnetic
field, and a higher heat resistant temperature.
Another object of the present invention is to provide a high
performance MR device, typically a MR head capable of reading high
density record information using the magnetic multilayer film.
According to the present invention, there is provided a magnetic
multilayer film comprising a first magnetic layer and a second
magnetic layer having a greater coercive force than the first
magnetic layer. The first and second magnetic layers are
alternately stacked with a non-magnetic metal layer intervening
therebetween. The number of the first magnetic layers N1 and the
number of the second magnetic layers N2 fall in the range:
2.ltoreq.N1.ltoreq.4 and N2=N1-1. The first magnetic layer has a
thickness t1 of 10 to 80 .ANG., the second magnetic layer has a
thickness t2 of 20 to 90 .ANG., and the non-magnetic metal layer
has a thickness t3 of 20 to 90 .ANG..
In preferred embodiments, each first magnetic layer has a
magnetization M1 and each second magnetic layer has a magnetization
M2 wherein 0.3.ltoreq.M1/M2.ltoreq.0.8; the first magnetic layer
has an anisotropic magnetic field Hk of 3 to 20 Oe.
With respect to layer compositions, the present invention favors
that the first magnetic layer comprises a magnetic metal containing
at least 70% by weight of a composition of the formula: (Ni.sub.x
Fe.sub.1-x).sub.1-y Co.sub.y wherein x and y representative of
weight ratios are 0.7.ltoreq.x.ltoreq.0.9 and
0.ltoreq.y.ltoreq.0.3;
the second magnetic layer comprises a magnetic metal containing at
least 70% by weight of a composition of the formula: (Co.sub.z
Ni.sub.1-z).sub.w Fe.sub.1-w wherein z and w representative of
weight ratios are 0.4.ltoreq.z.ltoreq.1.0 and
0.5.ltoreq.w.ltoreq.1.0, or a magnetic metal containing at least
70% by weight of at least one element selected from the group
consisting of cobalt, iron, and nickel and having added thereto at
least one element selected from the group consisting of Pt, Pt, Tb,
Gd, Dy, Sm, Nd, Eu, and P; and
the nonimagnetic metal layer comprises a non-magnetic metal
containing at least one element selected from the group consisting
of gold, silver, and copper.
The magnetic multilayer film according to one preferred embodiment,
upon application of a DC magnetic field, provides a
magnetoresistance curve having a maximum slope of at least 0.2%/Oe
in a magnetic field in the range between -10 Oe and +10 Oe.
The magnetic multilayer film according to another preferred
embodiment, upon application of an AC magnetic field at a frequency
of 1 MHz and an amplitude of 10 Oe over the range between -20 Oe
and +20 Oe, provides a magneto-resistance curve having a maximum
slope of at least 0.1%/Oe.
In another aspect, the present invention provides a method for
preparing a magnetic multilayer film as defined above comprising
the steps of forming the first magnetic layer, the second magnetic
layer and the non-magnetic metal layer, wherein during the step of
forming the first magnetic layer, a magnetic field is applied in
one in-plane direction of the first magnetic layer.
Preferably, the steps of forming the first magnetic layer, the
second magnetic layer and the non-magnetic metal layer are achieved
by effecting evaporation such that particles being deposited may
possess an energy of 0.01 to 10 eV. Also preferably, the steps of
forming the first magnetic layer, the second magnetic layer and the
non-magnetic metal layer are achieved by effecting evaporation in
an atmosphere of up to 10.sup.-8 Torr.
The method may further include the step of heat treating the
resultant magnetic multilayer film at a temperature of up to
300.degree. C. Also preferably, the resultant magnetic multilayer
film is heat treated at a temperature of up to 400.degree. C. In an
atmosphere of up to 10.sup.-7 Torr.
In a further aspect, the present invention provides a
magnetoresistance device comprising a magnetic multilayer film as
defined above as a magneto-sensitive section. The magnetoresistance
device does not need a bias magnetic field applying mechanism.
In a field-induced ferrimagnetic multilayer film having magnetic
layers stacked with intervening non-magnetic metal layers, it is
still insufficient for achieving the above-mentioned objects to
limit such factors as coercivity difference and squareness ratio as
described in the above-referred articles and patents. The present
invention achieves the above-mentioned objects by taking advantage
of the soft magnetic properties of the first magnetic layers with
lower coercivity and optimizing the arrangement of two types of
magnetic layers.
Since the number of second magnetic layers is smaller by one than
the number of first magnetic layers, the overall thickness of the
magnetic multilayer film according to the present invention is
smaller than that of the previously mentioned multilayer film
wherein the number of recurring units N is 4 or less. Since the
number of first magnetic layers with lower coercivity is greater
than the number of second magnetic layers, the above-mentioned
objects are achieved even when the overall number of magnetic
layers stacked is small. In all the previously mentioned
field-induced ferrimagnetic multilayer films, two types of magnetic
layers having different coercive forces are used in an identical
number. In the previous field-induced ferrimagnetic multilayer
films, the construction that magnetic layers with lower coercivity
are stacked in a more number was not contemplated. It was, of
course, unknown that such a construction is effective especially
when a smaller number of magnetic layers are stacked.
A MR head using the magnetic multilayer film of the invention as a
magneto-sensitive section can produce an output voltage which is
about 3 times greater than MR heads using conventional magnetic
multilayer films. Accordingly the MR head of the invention ensures
high reliability readout and can read out signals magnetically
recorded at a ultrahigh density of 1 Gbit per square inches or
more.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a magnetic multilayer film
according to the invention.
FIG. 2 is a diagram showing exemplary B-H curves for explaining the
principle of the invention.
FIG. 3 is a fragmental cross-sectional view of a MR device using a
magnetic multilayer film as a magneto-sensitive section according
to one embodiment of the invention.
FIG. 4 is a fragmental cross-sectional view of a yoke type MR head
using a magnetic multilayer film as a magneto-sensitive section
according to another embodiment of the invention.
FIG. 5 is a fragmental cross-sectional view of a flux guide type MR
head using a magnetic multilayer film as a magneto-sensitive
section according to a further embodiment of the invention.
FIG. 6 diagrammatically illustrates an MR curve of a magnetic
multilayer film (sample No. 108) according to the invention under a
DC magnetic field.
FIG. 7 is a diagram showing the output voltage versus applied
magnetic field of a MR head using a magnetic multilayer film as a
magneto-sensitive section according to the invention.
BEST MODE FOR CARRYING OUT THE INVENTION
In the magnetic multilayer film according to the present invention,
first and Second magnetic layers having different coercive forces
are disposed adjacent to each other with a non-magnetic metal layer
interposed therebetween. The two types of magnetic layers must have
different coercive forces because the principle of the invention is
that conduction electrons are subject to spin-dependent scattering
to increase electrical resistance as adjacent magnetic layers are
offset in the direction of magnetization, and the resistance
reaches maximum when the adjacent magnetic layers have opposite
directions of magnetization.
Now, the function of the magnetic multilayer film according to the
present invention will be understood from the following description
of the relationship of an external magnetic field to the coercive
force and magnetization direction of respective magnetic layers of
a ternary magnetic multilayer film. For the sake of brevity of
description only, reference is made to a structure having only one
first magnetic layer and one second magnetic layer. Assume that
first and second magnetic layers (1) and (2) have different
coercive forces Hc.sub.1 and Hc.sub.2 (0<Hc.sub.1 <Hc.sub.2),
respectively, the first magnetic layer (1) has an anisotropic
magnetic field Hk, and the magnetization of the second magnetic
layer (2) is saturated at an external magnetic field Hm as shown in
FIG. 2. At the initial, an external magnetic field H is applied
wherein H<-Hm. The first and second magnetic layers (1) and (2)
have magnetization directions oriented in a negative (-) direction
same as H. Then the external magnetic field is increased to region
I of H<-Hk where both the magnetic layers have magnetization
directions oriented in one direction. As the external magnetic
field is increased to region II of -Hk<H<Hk, magnetic layer
(1) partially starts reversing its magnetization direction so that
the magnetization directions of magnetic layers (1) and (2) may
include opposite components. The magnetization directions of
magnetic layers (1) and (2) are in substantially complete
anti-parallelism in the range of Hk<H<Hc2. When the external
magnetic field is further increased to region III of Hm<H,
magnetic layers (1) and (2) have magnetization directions aligned
in a positive (+) direction.
Now, the external magnetic field H is reduced. In region IV of
Hk<H, the magnetic layers (1) and (2) have magnetization
directions still aligned in a positive (+) direction. In region V
of -Hk<H<+Hk, the magnetic layer (1) starts reversing its
magnetization direction in one direction so that the magnetization
directions of magnetic layers (1) and (2) may include opposite
components. Subsequently in region VI of H<-Hm, the magnetic
layers (1) and (2) have magnetization directions aligned in one
direction again. In the regions II and V where the magnetic layers
(1) and (2) have opposite magnetization directions, conduction
electrons are subject to spin-dependent scattering, resulting in an
increased resistance. In the zone of -Hk<H<Hk in region II,
magnetic layer (2) undergoes little magnetization reversal, but
magnetic layer (1) linearly increases its magnetization, the
proportion of conduction electrons subject to spin-dependent
scattering is gradually increased in accordance with a
magnetization change of magnetic layer (1). By selecting a low Hc
material such as Ni.sub.0.8 Fe.sub.0.2 (Hc.sub.1 =several Oe) as
the first magnetic layer (1), imparting appropriate Hk thereto and
selecting a somewhat high Hc, high squareness ratio material such
as Co (Hc.sub.2 =several tens of Oe) as the second magnetic layer
(2), for example, there is obtained an MR device exhibiting a
linear MR change and a great MR ratio in a low external magnetic
field in the range of several oersteds to several tens of oersted
near or below Hk.
Referring to FIG. 1, there is shown in cross section a magnetic
multilayer film 1 according to one embodiment of the invention. The
magnetic multilayer film 1 is formed on a metal underlying layer 10
on a substrate 0 while a protective layer 80 is formed on the
magnetic multilayer film 1. The magnetic multilayer film 1 includes
a lower first magnetic layer 20, a lower non-magnetic metal layer
30, a second magnetic layer 40 having a greater coercive force than
the first magnetic layer, an upper non-magnetic metal layer 30, and
an upper first magnetic layer 20 stacked from bottom to top in the
described order. It is understood that a non-magnetic metal layer
may intervene between the metal underlying layer 10 and the lower
first magnetic layer 20.
In the magnetic multilayer film of the invention, provided that N1
is the number of the first magnetic layers having a smaller
coercive force and N2 is the number of the second magnetic layers
having a greater coercive force, N1 and N2 fall in the range:
Under an external magnetic field having a strength of the order of
signal magnetic field, the second magnetic layer having a greater
coercive force does not change the direction of spin and has a
magnetic spin structure fixed in a certain direction. On the other
hand, the first magnetic layer having a smaller coercive force does
change the direction of spin even under a weak external magnetic
field having a strength of the order of signal magnetic field. As a
consequence, the magnetic multilayer film exhibits a greater
resistance change. Since the number of first magnetic layers
susceptible to spin rotation under a weak magnetic field is more,
the magnetic multilayer film according to the invention provides a
greater MR change although it is thin. More particularly, since
conduction electrons undergo most efficient scattering by magnetic
spin while they flow through the magnetic multilayer film, a great
MR effect is obtained as compared with prior art magnetic
multilayer films wherein N1=N2. In general, the MR effect is
reduced as the number of magnetic layers stacked is reduced. For
example, the magnetic multilayer film described in the
above-referred article (b) has an MR ratio as low as 4% for N1=N2=3
and 1.2% for N1=N2=1 provided that the numbers of magnetic layers
are expressed by N1 and N2 as in the present invention. In contrast
to article (b) wherein N1=N2, the present invention ensures a
greater MR ratio despite a smaller number of magnetic layers by
setting N2=N1-1. As will be described later, by inducing an
anisotropic magnetic field Hk of 1 to 20 Oe in the first magnetic
layer having a smaller coercive force, there is obtained a magnetic
multilayer film providing a MR curve with a greater slope in
proximity to zero magnetic field and having a satisfactory MR
effect at high frequency. If N1 is more than 4, no outstanding
effects are achieved despite the setting: N2=N1-1.
The long period structure can be observed by taking a small angle
X-ray diffraction pattern where primary and secondary peaks
corresponding to recurring periodicities appear.
In the invention, the respective magnetic layers have coercive
forces Hc which may be suitably selected in the range of, for
example, 0.001 Oe to 10 kOe, especially 0.01 to 1000 Oe, depending
on the strength of an applied external magnetic field and the MR
ratio required for the device associated therewith. The ratio in
coercive force between first and second magnetic layers, Hc.sub.2
/Hc.sub.1 is preferably from 1.2:1 to 100:1, especially from 1.5:1
to 100:1, more preferably from 2:1 to 80:1, especially from 3:1 to
60:1, most preferably from 5:1 to 50:1, especially from 6:1 to
30:1. Outside the range, a higher Hc.sub.2 /Hc.sub.1 ratio would
result in a broader MR curve whereas a lower ratio leads to a
smaller difference between coercive forces, failing to take
advantage of anti-parallelism.
Coercive force Hc is measured as follows because it is impossible
to directly measure the magnetic properties of magnetic layers in a
magnetic multilayer film. For example, first magnetic layers to be
measured for Hc are deposited by evaporation alternately with
non-magnetic metal layers until the total thickness of the magnetic
layers reaches about 200 to 400 .ANG.. The resulting sample is
measured for magnetic properties. It is to be noted that the
thickness of magnetic layers, the thickness and composition of
non-magnetic metal layers, and their deposition method are the same
as in a magnetic multilayer film to be examined.
In the magnetic multilayer film of the invention, there are
included a plurality of first magnetic layers and one or more
second magnetic layers. For each type, a plurality of magnetic
layers have substantially the same coercive force because their
composition and deposition method are generally unchanged. Since
only Hc.sub.1 <Hc.sub.2 is required in the magnetic multilayer
film of the invention, a plurality of first magnetic layers need
not necessarily have an identical coercive force Hc.sub.1 and
similarly, a plurality of second magnetic layers need not
necessarily have an identical coercive force Hc.sub.2.
In order to provide an MR curve having good linearity across zero
magnetic field and improved heat resistance, the respective
magnetic layers should preferably have controlled squareness ratio
SQ=residual magnetization Mr/saturated magnetization Ms. The first
magnetic layer should preferably have a squareness ratio SQ.sub.1
of 0.01.ltoreq.SQ.sub.1 .ltoreq.0.5, more preferably
0.01.ltoreq.SQ.sub.1 .ltoreq.0.4, most preferably
0.01.ltoreq.SQ.sub.1 .ltoreq.0.3. The second magnetic layer should
preferably have a squareness ratio SQ.sub.2 of 0.7.ltoreq.SQ.sub.2
.ltoreq.1.0. since the first magnetic layer governs the rise of MR
change in the vicinity of zero magnetic field, its squareness ratio
SQ.sub.1 is preferably as small as possible. More particularly,
with smaller SQ.sub.1, magnetization will gradually rotate and
anti-parallelism will gradually increase in the vicinity of zero
magnetic field, resulting in a linear MR curve across zero magnetic
field. With SQ.sub.1 larger than 0.5, it is difficult to provide a
linear MR change. The lower limit of SQ.sub.1 from the
manufacturing aspect is about 0.01. The second magnetic layer
should preferably have a squareness ratio SQ.sub.2 close to 1 in
the vicinity of zero magnetic field. With a squareness ratio
SQ.sub.2 of 0.7 or higher, the rise of MR change in the vicinity of
zero magnetic field becomes sharp and a great MR ratio is
obtainable. Preferably SQ.sub.2 /SQ.sub.1 is between 2 and 100,
especially between 2 and 50.
The composition of the first magnetic layer with smaller coercive
force is not critical although it preferably comprises a magnetic
metal containing at least 70% by weight of a composition of the
formula:
wherein x and y representative of weight ratios of Ni and Co are
0.7.ltoreq.x.ltoreq.0.9 and 0.ltoreq.y.ltoreq.0.3. Preferably the
first magnetic layer consists essentially of this magnetic metal.
Use of this magnetic metal provides magnetic layers with a smaller
coercive force and satisfactory soft magnetic properties. As a
result, a MR change curve having a sharp rise and high magnetic
field sensitivity are available. With x and y outside the range,
the resulting magnetic layer has a greater coercive force, failing
to achieve a high magnetic field sensitivity.
Also the composition of the second magnetic layer with greater
coercive force is not critical although it preferably comprises a
magnetic metal containing at least 70% by weight of a composition
of the formula:
wherein z and w representative of weight ratios of Co and Co-Ni are
0.4.ltoreq.z.ltoreq.1.0 and 0.5.ltoreq.w.ltoreq.1.0. Preferably the
second magnetic layer consists essentially of this magnetic metal.
Use of this magnetic metal provides second magnetic layers with
satisfactory magnetic properties relative to the first magnetic
layer. Examples of this magnetic metal include Co, CoFe, CoNi, and
CoFeNi. For the second magnetic layer, use may also be made of a
magnetic metal containing at least 70% by weight of at least one
element selected from Co, Fe, and Ni, and further containing at
least one subordinate element selected from the group consisting of
Pr, Pt, Tb, Gd, Dy, Sm, Nd, Eu, and P. Addition of the subordinate
element increases the coercive force of the second magnetic layer,
stabilizes a linear rise portion of a MR change curve at zero
magnetic field, and increases the stability against external
disturbing magnetic field. Examples of the magnetic metal having
such a subordinate element added include CoPt, CoP, CoNiP, CoGd,
CoTb, CoDy, CoSm, CoPr, CoNd, FeTb, FeGd, FeDy, FeNd, FeSm, FeEu,
FeCoTb, FeCoPt, and FeCoGdDy.
The non-magnetic metal layer is preferably formed of a conductive
metal for effectively conduction electrons, typically a
non-magnetic metal containing at least one element selected from
gold (Au), silver (Ag), and copper (Cu). Preferably the sum of Au,
Ag, and Cu occupies at least 60% by weight of the non-magnetic
metal layer.
For application to MR heads, the respective layers of the magnetic
multilayer film should preferably be as thin as possible because
the shield length is reduced to allow for reading-out of high
density signals. However, if too thin, soft magnetic, ferromagnetic
or anti-ferromagnetic properties required for the respective layers
are lost and heat resistance is poor. While the present invention
is characterized by a reduced number of magnetic layers, no
satisfactory MR effect is accomplished unless the respective
magnetic layers perform well. Since the magnetostatic bond and
direct exchange interaction between respective layers are
substantially reduced in a magnetic multilayer film having a small
number of magnetic layers, the magnetic layers may be made thick as
compared with a magnetic multilayer film having a large number of
magnetic layers and an increased total thickness. If the respective
magnetic layers are too thick, however, the probability of
conduction electrons being scattered becomes low at a certain
relative angle between spins of the two types of magnetic layers
and the MR effect is rather reduced. Increased resistivity is
inconvenient for multilayer application. If a non-magnetic metal
layer is too thick, most conduction electrons pass through this
non-magnetic metal layer and a smaller proportion of conduction
electrons are scattered in magnetic layers, resulting in a lower
percent MR ratio. Conversely, if a non-magnetic metal layer is too
thin, a greater magnetic interaction occurs between magnetic
layers, with the likelihood of the two magnetic layers having
different magnetization directions being lost. For this and other
reasons, the present invention limits the thickness of the
respective layers. The first magnetic layer has a thickness t1, the
second magnetic layer has a thickness t2, and the non-magnetic
metal layer has a thickness t3 wherein
preferably 20 .ANG..ltoreq.t1, t2, t3.ltoreq.80 .ANG., more
preferably 20 .ANG..ltoreq.t1, t2, t3.ltoreq.70 .ANG..
Understandably, the thickness of magnetic layers and non-magnetic
layers can be measured by means of a transmission electron
microscope or scanning electron microscope or by Auger electron
spectroscopy. The crystal structure of layers can be observed by
X-ray diffraction or high speed electron diffraction analysis.
It is the spins of both the first and second magnetic layers that
contributes to scattering of conduction electrons. Most efficient
scattering occurs when both the spins are of substantially the same
magnitude, that is, when the total quantity of magnetization a
number (N1) of first magnetic layers possess is approximately equal
to the total quantity of magnetization a number (N2) of second
magnetic layers possess. Because of N1>N2, more efficient
scattering occurs when the quantity of magnetization of a single
first magnetic layer is smaller than the quantity of magnetization
of a single second magnetic layer. More particularly, more
efficient scattering of conduction electrons occurs when the
respective magnetic layers are preferably adjusted in thickness so
as to meet 0.3.ltoreq.M1/M2.ltoreq.0.8, more preferably
0.4=M1/M2.ltoreq.0.8 provided that a single first magnetic layer
has a quantity of magnetization M1 and a single second magnetic
layer has a quantity of magnetization M2. With this adjustment, the
region of the magnetic multilayer film which does not contribute to
scattering of conduction electrons is reduced. Understandably,
M1/M2 is obtained by multiplying the magnetization (magnetic moment
per unit volume) of each magnetic layer by its thickness and
dividing the product for the first magnetic layer by the product
for the second magnetic layer. The magnetization of each magnetic
layer is measured using the above-mentioned sample for coercive
force measurement. Where all of the first magnetic layers are not
equal in thickness, the magnetization per layer is determined using
an average thickness of the first magnetic layers. This is also
true for the second magnetic layers.
The first magnetic layer should preferably be imparted an
anisotropic magnetic field Hk of 3 to 20 Oe, more preferably 3 to
16 Oe, most preferably 3 to 12 Oe. Such Hk can be imparted to the
first magnetic layer by forming the layer while applying an
external magnetic field in one direction in its plane. If the
anisotropic magnetic field Hk of the first magnetic layer is less
than 3 Oe, it is approximate to the coercive force, and the
multilayer film would not provide an MR curve which is linear
across zero magnetic field or satisfy the MR head requirements. If
the first magnetic layer's Hk is more than 20 Oe, the multilayer
film would have a reduced MR slope so that MR heads constructed
therefrom will provide low outputs and low resolution. The external
magnetic field used for imparting Hk is preferably of 10 to 300 Oe.
An external magnetic field of less than 10 Oe would be difficult to
impart the desired Hk whereas an external magnetic field of more
than 300 Oe would provide no further merits in imparting Hk and
requires a larger coil to generate it, leading to an increased
expense and less efficiency. An external magnetic field may be
applied only when the first magnetic layer is formed.
Alternatively, an external magnetic field may be applied throughout
the process for the manufacture of a magnetic multilayer film
including formation of other layers. In the former case wherein an
external magnetic field is applied only during formation of the
first magnetic layers, the system may be provided with means
capable of applying a magnetic field at controlled timing, for
example, an electromagnet. Understandably, the anisotropic magnetic
field can be measured using the above-mentioned sample for coercive
force measurement.
In the magnetic multilayer film of the invention, a rise portion of
the MR curve preferably has a gradient (or MR slope) of at least
0.2%/Oe, more preferably at least 0.25%/Oe, especially 0.3 to
1.0%/Oe in an external magnetic field between -10 Oe and +10 Oe.
The MR slope is obtained by measuring an MR to depict an MR curve,
determining differential values therefrom, and determining a
maximum differential value over the magnetic field range between
-10 Oe and +10 Oe. It is to be noted that the MR ratio of a
magnetic multilayer film under a DC magnetic field having a
strength H is calculated as
wherein .rho..sub.H is the resistivity under an external magnetic
field of strength H and .rho..sub.sat is the resistivity when all
the magnetic layers of the magnetic multilayer film are saturated
in magnetization (that is, minimum resistivity). The .rho..sub.sat
of the magnetic multilayer film according to the invention is
determined in an external magnetic field over the range between
-300 Oe and +300 Oe.
Also in the magnetic multilayer film of the invention, the MR
hysteresis curve obtained by applying a magnetic field of about
-300 Oe for once saturating all the magnetic layers and sweeping a
magnetic field in the range between -20 Oe and +20 Oe to depict a
minor loop can have a maximum width of up to 8 Oe, especially 0 to
6 Oe.
Further, the slope of a MR curve (or high-frequency MR slope)
obtained by measuring an MR ratio in an alternating magnetic field
having an amplitude of 10 Oe and a frequency of 1 MHz and
determining a gradient thereof between -20 Oe and +20 Oe can be at
least 0.1%/Oe, more preferably at least 0.15%/Oe, most preferably
0.2 to 1.0%/Oe. That is, the desirable high-frequency MR slope can
be obtained independent of whether a DC bias magnetic field in the
range between -15 Oe and +15 Oe is applied or not. Then when the
magnetic multilayer film is applied to MR heads for high density
record reading, satisfactory performance is expectable. It is noted
that the high-frequency MR slope is also a maximum of the
differential values of a MR curve wherein the MR ratio is given
by
wherein .rho..sub.min is a minimum resistivity upon application of
a high-frequency magnetic field having an amplitude of 10 Oe.
The magnetic multilayer film can be formed by conventional methods
such as ion beam sputtering, sputtering, evaporation, and molecular
beam epitaxy (MBE) methods. Since the magnetic multilayer film of
the invention has a magnetization structure of the field-induced
ferrimagnetic type, layer deposition should be done so as to
minimize the intermixing of elements between adjacent layers at
each interface in a multilayer structure. To this end, evaporation
is preferably carried out to form each layer such that particles
being deposited may have an energy of 0.01 to 10 eV, more
preferably 0.05 to 10 eV, preferably with a median energy of 0.1 to
8 eV, especially 0.1 to 6 eV. If the depositing particles have too
much energy, substantial mixing of different elements can occur at
the interface to form an alloy, failing to provide a significant MR
effect of the induction ferrimagnetic type. If the depositing
particles have too low energy, the resulting magnetic multilayer
film would have low crystallinity and high resistivity, failing to
provide a significant MR effect. Further preferably, evaporation is
effected in a vacuum of up to 10.sup.-8 Torr. A magnetic multilayer
film of quality can also be formed by a method other than
evaporation, for example, by ion beam sputtering insofar as the
energy of depositing particles is properly controlled.
The substrate on which the magnetic multilayer film is formed may
be of glass, silicon, magnesium oxide (MgO), gallium arsenide
(GaAs), ferrite, AlTiC and CaTiO.
Optionally the metal underlying layer is provided for the purposes
of mitigating the difference in surface energy between the material
of the magnetic multilayer film and the substrate material and
improving the wetting therebetween for accomplishing a laminate
structure having a flat interface over a wide area. The material of
which the metal underlying layer is made is not critical and
includes Cr, Ta, Hf, Cu, Au, Ag, Nb, and Zr and an alloy containing
at least one of them. The metal underlying layer is generally about
10 to 100 .ANG. thick.
The protective layer is provided for protecting and preventing
oxidation of the magnetic multilayer film. The protective layer is
generally constructed by various dielectric materials such as
silicon nitride, silicon oxide, and aluminum oxide.
A process for manufacturing an MR head using the magnetic
multilayer film according to the invention necessarily involves
heat treatment such as baking, annealing and resist curing for
patterning, flattening and the like. In general, a problem of heat
resistance often arises with magnetic multilayer films which are
referred to as artificial superlattices because they are
constructed by thin layers. However, the magnetic multilayer film
of the invention can withstand heat treatment of up to 300.degree.
C. and 2 hours by imparting a desirable anisotropic magnetic field
to the first magnetic layers. Heat treatment is generally carried
out in vacuum, inert gas atmosphere or air. Especially when heat
treatment is carried out in a vacuum of up to 10.sup.-7 Torr, the
magnetic multilayer film is minimized deterioration and tolerant to
heat treatment of up to 400.degree. C. By imparting an anisotropic
magnetic field, any deterioration of MR effect by lapping and
polishing is avoidable.
In general, MR heads using Permalloy include a shunt layer of Ti or
the like or a bias magnetic field applying layer of high
resistivity soft magnetic material such as CoZrMo and NiFeRh, both
disposed adjacent the magneto-sensitive section. These layers
constitute a bias magnetic field applying mechanism of shifting an
MR curve of Permalloy to develop a linear region centering at zero
magnetic field, which mechanism is referred to as shunt bias or
soft film bias. Due to complexity, however, this mechanism becomes
a factor of substantially reducing the yield of the manufacture
process. Since the MR curve of the magnetic multilayer film
according to the invention rises from the very vicinity to zero
magnetic field, a linear region centering at zero magnetic field
can be developed by self biasing resulting from current flow
through the magnetic multilayer film and a Shift of the MR curve
due to a diamagnetic field resulting from the pattern
configuration. As a consequence, a need for a bias magnetic field
applying mechanism is eliminated, leading to the advantages of an
increased manufacturing yield, a shortened manufacturing time, and
a cost reduction. Since the magneto-sensitive section is reduced in
thickness due to the absence of a bias magnetic field applying
mechanism, the resulting MR head has a reduced shield length, which
is very effective for reducing the wavelength of signals for
ultrahigh density recording.
Referring to FIG. 3, there is illustrated one particular embodiment
of the present invention wherein the magnetic multilayer film is
applied to a magneto-sensitive section of an MR device such as MR
head. The MR device shown in FIG. 3 includes a magnetic multilayer
film 1 formed within a non-magnetic insulating layer 400 and a pair
of electrodes 100 and 100 connected to opposite sides of the
magnetic multilayer film 1 for conducting measuring electric
current across the magnetic multilayer film 1. The electrodes 100
are usually formed of Cu, AG, Au, W, and Ta, for example. The
non-magnetic insulating layer 400 is usually formed of oxides
commonly used to form non-magnetic insulating layers, for example,
SiO.sub.2, SiO, and Al.sub.2 O.sub.3. The non-magnetic insulating
layer 400 and hence the magnetic multilayer film 1 is sandwiched
between a pair of shields 300 and 300 of Sendust or Permalloy.
FIG. 4 illustrates another particular embodiment of the present
invention wherein the magnetic multilayer film is applied to a
magneto-sensitive section of a yoke type MR head. The
magneto-sensitive section includes first and second yokes 601 and
602 which at one end are closely spaced from each other and opposed
to a magnetic recording medium and at another end remote from the
medium (upper end as viewed in FIG. 4) are disposed in contact. The
first yoke. 601 consists of a lower yoke segment and an upper yoke
segment which are not magnetically coupled. Disposed between the
lower and upper yoke segments is a magnetic multilayer film 1
having an in-plane direction substantially aligned with the first
and second yokes. A non-magnetic insulating layer 400 intervenes
between the yokes and the magnetic multilayer film 1. The magnetic
multilayer film 1 is provided with electrodes (not shown) for
conducting electric current flow parallel to or perpendicular to
the direction of a magnetic path generated by the yokes.
FIG. 5 illustrates a further particular embodiment of the present
invention wherein the magnetic multilayer film is applied to a
magneto-sensitive section of a flux guide type MR head. The
magneto-sensitive section includes a high-resistivity flux guide
layer 800 and a flux guide layer 700 disposed so as to face a
magnetic recording medium at one end. The high-resistivity flux
guide layer 800 consists of a high-resistivity flux guide layer
lower segment and a high-resistivity flux guide layer upper
segment, between which a magnetic multilayer film 1 intervenes. The
magnetic multilayer film 1 has opposite ends in its in-plane
direction magnetically coupled to the upper and lower
high-resistivity flux guide layer segments. The magnetic multilayer
film 1 is also provided with electrodes (not shown) for conducting
electric current flow parallel to or perpendicular to the direction
of a magnetic path. A non-magnetic insulating layer 400 intervenes
between the flux guide layer 700 and the magnetic multilayer film 1
and high-resistivity flux guide layer 800. The flux guide layer 700
functions as a return guide for a magnetic flux passing past the
magnetic multilayer film 1. Since the high-resistivity flux guide
layer 800 is formed of a material having a higher resistivity than
the magnetic multilayer film 1 by a factor of 3 or more, for
example, CoZr, CoZrNb, NiFeRh, FeSiB, and CoZrMo, the measuring
current across the magnetic multilayer film 1 does not flow across
the high-resistivity flux guide layer 800 in a substantial sense.
On the other hand, since the high-resistivity flux guide layer 800
is magnetically coupled to the magnetic multilayer film 1, a signal
magnetic field induced in the high-resistivity flux guide layer 800
lower segment reaches the magnetic multilayer film 1 without a loss
of its strength. In the illustrated embodiment, the flux guide
layer 700 is disposed on only one side of the magnetic multilayer
film 1 although a pair of flux guide layers may be provided so as
to sandwich the magnetic multilayer film 1 therebetween. Also, the
flux guide layer 700 may be connected to the high-resistivity flux
guide layer upper segment at the end remote from the medium (upper
end as viewed in FIG. 5).
Optionally the magnetic multilayer film of the invention may be
provided with a shunt layer or bias magnetic field applying layer
although they are unnecessary as mentioned previously.
EXAMPLE
Examples of the present invention are given below by way of
illustration and not byway of limitation.
Example 1
A glass substrate was placed in a ultrahigh vacuum evaporation
chamber which was evacuated to a vacuum of 1.times.10.sup.-10 to
3.times.10.sup.-10 Torr. While rotating the substrate at room
temperature, a magnetic multilayer film of the following
composition was formed on the substrate by first depositing a
chromium layer of 50 .ANG. thick as an underlying layer, and then
depositing a first magnetic layer, non-magnetic metal layer, and
second magnetic layer through evaporation. Deposition conditions
included a pressure of 1.1.times.10.sup.-9 Torr and an energy
depositing particles-possessed of 0.06 to 2 eV with a median energy
of 0.2 eV. During deposition, a magnetic field having the strength
shown in Table 1 was applied in one direction in a plane
coextensive with the substrate and parallel to the measuring
current flow. The film growth rate for each layer was about 0.3
A/sec.
Table 1 shows the construction of the respective layers. The
material and thickness of the respective layers are designated by
m1 and t1 for the first magnetic layer, m2 and t2 for the second
magnetic layer, and m3 and t3 for the non-magnetic metal layer and
reported in the order of (m1, m2, m3) and (t1, t2, t3),
respectively. NiFe for m1 stands for the Permalloy composition of
80 wt % Ni-Fe. The number N1 of the first magnetic layers and the
number N2 of the second magnetic layers are also reported in Table
1. For those samples wherein N2=N1-1, evaporation was started with
the first magnetic layer, both the magnetic layers were alternately
evaporated while interposing a non-magnetic metal layer between
them, and evaporation was terminated with the first magnetic layer.
For those samples wherein N2=N1, the process was the same except
that the last evaporation of the first magnetic layer was omitted.
For those samples wherein N2=N1+1, evaporation was started with the
second magnetic layer and terminated with the second magnetic
layer.
The ratio of the magnetization M1 per layer of the first magnetic
layers to the magnetization M2 per layer of the second magnetic
layers, M1/M2, and the anisotropic magnetic field Hk of the first
magnetic layer were determined by preparing special samples for
measurement as previously mentioned. A disc sample having a
diameter of about 10 mm was used for the measurement of Hk by means
of a magnetic torque meter. The results are shown in Table 1.
The samples in Table 1 were heat treated in a vacuum of 10.sup.-5
Tort at 230.degree. C. for 4 hours before the measurement of the
following properties. The results are also shown in Table 1.
Minimum resistivity (.rho..sub.sat)
Each of the samples in Table 1 was cut into a strip of 0.5
mm.times.10 mm, which was measured for resistance by a four
terminal method. For measurement, electric current was
longitudinally passed through the strip and an external magnetic
field was applied in plane and perpendicular to the electric
current and varied from -300 Oe to +300 Oe. From the resistance
measurement, minimum resistivity .rho..sub.sat was determined.
Maximum MR ratio (maximum MR) and slope of MR curve (MR slope)
The resistivity .rho..sub.H was measured under an external magnetic
field H over the range from -300 Oe to +300 Oe. The percent MR
ratio .DELTA.R/R was calculated according to the equation:
to determine its maximum (maximum MR). An MR hysteresis curve was
depicted as shown in FIG. 6 and differentiated to determine a
maximum (MR slope) among the magnitudes of differential values
within the H range between -10 Oe and 10 Oe, for rise evaluation.
The MR curve in FIG. 6 is of sample No. 108 (without heat
treatment)
Slope of MR curve at high frequency (high-frequency MR slope)
A percent MR ratio was determined by applying a high-frequency
magnetic field at 1 MHz and between -5 Oe and +5 Oe as an external
magnetic field, from which an MR curve was depicted and
differentiated to determine a maximum (high-frequency MR slope)
among the magnitudes of differential values.
TABLE 1
__________________________________________________________________________
Layer Magnetic Layer Thickness Number of field applied Heat
High-frequency Sample Material (.ANG.) Layers during deposition Hk
Treatment MR slope No. (m.sub.1, m.sub.2, m.sub.3) (t.sub.1,
t.sub.2, t.sub.3) N.sub.1 N.sub.2 M.sub.1 /M.sub.2 (Oe) (Oe)
(.degree.C.) (%/Oe)
__________________________________________________________________________
101 Comp. (NiFe, Co, Cu) (50, 50, 50) 5* 5* 0.75 90 6.5 230 0.15
102 Comp. (NiFe, Co, Cu) (50, 50, 50) 5* 4* 0.75 90 6.5 230 0.13
103 Comp. (NiFe, Co, Cu) (50, 50, 50) 4 4* 0.75 90 6.5 230 0.15 104
(NiFe, Co, Cu) (50, 50, 50) 4 3 0.75 90 6.5 230 0.42 105 Comp.
(NiFe, Co, Cu) (50, 50, 50) 3 3* 0.75 90 6.5 230 0.21 106 (NiFe,
Co, Cu) (50, 50, 60) 3 2 0.75 90 6.5 230 0.46 107 Comp. (NiFe, Co,
Cu) (50, 50, 50) 2 2* 0.75 90 6.5 230 0.20 108 (NiFe, Co, Cu) (50,
50, 50) 2 1 0.75 90 6.5 230 0.45 109 Comp. (NiFe, Co, Cu) (50, 50,
50) 1* 1* 0.75 90 6.5 230 0.05 110 Comp. (NiFe, Co, Cu) (50, 50,
50) 1* 2* 0.75 90 5.3 230 0.08
__________________________________________________________________________
*Outside the scope of the invention
As is evident from Table 1, the setting 2.ltoreq.N1.ltoreq.4 and
N2=N1-1 ensures a very large high-frequency MR slope even after
heat treatment. In contrast, despite N2=N1-1, if N1.ltoreq.5, then
the high-frequency MR slope is small as found with sample No. 102
and because of the increased thickness, the magnetic multilayer
film is unsuitable as a reading MR head for high density recording
media. Also despite 2.ltoreq.N1.ltoreq.4, if N2=N1, then the
high-frequency MR slope is small as found with sample Nos. 103,
105, and 107
Example 2
Following the procedure of Example 1, magnetic multilayer film
samples as shown in Table 2 were prepared. It is noted in Table 2
that for m1, NiFe is Permalloy of the same composition as in
Example 1 and NiFeCo is 80 wt % Ni-10 wt % Fe-Co; for m2, CoFe is
80 wt % Co-Fe, CoNi is 90 wt % Co-Ni, CoPt is 90 wt % Co-Pt, and
CoSm is 78 wt % Co-Sm; for m3, CuAu is 90 wt % Cu-Au. These samples
were evaluated as in Example 1. Evaluation was made on the samples
both before and after heat treatment under the same conditions as
in Example 1. The results from the samples before heat treatment
are reported in Table 3 and the results from the samples after heat
treatment are reported in Table 4.
Some of the samples in Table 1 were also evaluated before heat
treatment. They are also reported in Tables 3 and 4.
TABLE 2
__________________________________________________________________________
Layer Thickness Number of field applied Sample Material (.ANG.)
Layers during deposition Hk No. (m.sub.1, m.sub.2, m.sub.3)
(t.sub.1, t.sub.2, t.sub.3) N.sub.1 N.sub.2 M.sub.1 /M.sub.2 (Oe)
(Oe)
__________________________________________________________________________
108 (NiFe, Co, Cu) (50, 50, 50) 2 1 0.75 90 6.5 201 (NiFe, Co, Cu)
(40, 40, 50) 2 1 0.75 150 6.2 202 (NiFe, Co, Cu) (30, 30, 50) 2 1
0.75 90 5.9 203 (NiFe, CoFe, Cu) (60, 50, 40) 3 2 0.72 90 6.7 106
(NiFe, Co, Cu) (50, 50, 50) 3 2 0.75 90 6.5 204 (NiFe, CoNi, CuAu)
(50, 40, 60) 3 2 0.72 50 5.9 205 (NiFeCo, CoSm, Cu) (50, 60, 45) 3
2 0.78 90 7.6 206 (NiFeCo, CoPt, Cu) (50, 60, 45) 2 1 0.71 90 7.8
207 Comp. (NiFe, Co, Cu) (8, 8, 50)* 2 1 0.75 50 0* 208 Comp.
(NiFe, Co, Cu) (80, 95, 90)* 2 1 0.63 90 7.1 110 Comp. (NiFe, Co,
Cu) (50, 50, 50) 1* 2* 0.75 90 5.3
__________________________________________________________________________
*Outside the scope of the invention
TABLE 3 ______________________________________ High- Heat Maximum
MR frequency Sample treatment .rho.sat MR slope MR slope No.
(.degree.C.) (.mu..OMEGA. cm) (%) (%/Oe) (%/Oe)
______________________________________ 108 none 16.8 5.8 0.51 0.46
201 none 15.9 5.1 0.42 0.36 202 none 15.3 4.6 0.32 0.28 203 none
12.4 4.8 0.39 0.31 106 none 16.8 5.8 0.55 0.48 204 none 10.4 3.1
0.30 0.23 205 none 20.6 5.1 0.41 0.32 206 none 19.2 5.5 0.40 0.33
207 C none 10.2 0.2 0.01 0 208 C none 25.2 1.1 0.11 0.03 110 C none
17.6 4.2 0.19 0.13 ______________________________________ C:
comparison
TABLE 4 ______________________________________ High- Heat Maximum
MR frequency Sample treatment .rho.sat MR slope MR slope No.
(.degree.C.) (.mu..OMEGA. cm) (%) (%/Oe) (%/Oe)
______________________________________ 108 230 17.2 5.6 0.52 0.45
201 230 16.3 5.0 0.41 0.34 202 230 15.5 4.3 0.30 0.28 203 230 12.8
4.4 0.35 0.28 106 230 17.2 5.5 0.54 0.46 204 230 10.9 3.0 0.30 0.24
205 230 21.0 5.0 0.38 0.30 206 230 19.5 5.2 0.39 0.31 207 C 230
15.6 0.1 0 0 208 C 230 27.3 0.8 0 0 110 C 230 19.3 3.6 0.14 0.08
______________________________________ C: comparison
The effectiveness of the invention is evident from Tables 2 to 4.
For example, sample No. 108 before heat treatment has a maximum MR
ratio of 5.8% and a MR slope of 0.51%/Oe and its MR curve exhibits
a steep rise near zero magnetic field as shown in FIG. 6. In
comparison with the sample of the previously mentioned article (b)
having a maximum MR ratio of 4% for N1=N2=3 and 1.2% for
N1=N2=1.2%, the present invention provides substantial improvements
in MR effect.
As long as the respective magnetic layers and non-magnetic metal
layers have a thickness within the scope of the present invention,
a great maximum MR ratio and large MR slope were achieved. Good
results were obtained when the magnetic layers and non-magnetic
metal layers were changed in composition. Apart from the
composition shown in the Tables, equivalent results are obtainable
from various compositions selected from the previously mentioned
preferable composition.
A comparison of Table 3 with Table 4 reveals that the magnetic
multilayer films of the invention underwent little deterioration by
heat treatment. It is noted that upon heat treatment at a
temperature of 350.degree. C., the MR slope was maintained large if
the heat treatment was under a pressure of 10.sup.-7 Torr or less.
However, if the heat treatment was under a pressure of higher than
10.sup.-7 Torr, the MR slope was substantially reduced with an
increasing pressure because the magnetic multilayer film was
oxidized by a trace amount of residual oxygen.
An additional magnetic multilayer film was prepared by the same
procedure as sample No. 203 except that thickness (t1, t2, t3) was
changed to (70, 30, 40). It had a M1/M2 ratio as high as 1.40 and
as a consequence, an MR slope which was 30% smaller than that of
sample No. 108.
Example 3
On an AlTiC substrate, a shield layer of Sendust and a non-magnetic
insulating layer of Al.sub.2 O.sub.3 were formed and a chromium
layer of 50 .ANG. thick was formed as a metal underlying layer. A
magnetic multilayer film according to the invention was formed
thereon. The film had the same construction as sample No. 108 in
Table 1. Deposition conditions were approximately the same as
sample No. 108 except for some changes including a vacuum chamber
ultimate pressure of 1.3.times.10.sup.-10 Tort, a pressure during
deposition of 1.2.times.10.sup.-9 Torr, a substrate temperature of
about 35.degree. C., and a growth rate of 0.2 to 0.3 A/sec. By
photolithography, the magnetic multilayer film was patterned to
dimensions of 20 .mu.m.times.6 .mu.m, on which a gold electrode
having a track width of 3 .mu.m was formed. Further a non-magnetic
insulating layer of Al.sub.2 O.sub.3 and a shield layer of Sendust
were formed thereon to form an MR head. The thus prepared
magneto-sensitive section had a structure as shown in FIG. 3. A
measuring current of 15 mA was passed across the MR head while an
external magnetic field varying between -20 Oe and +20 Oe at 50 Hz
was applied. A change of the output voltage of the MR head is
depicted in FIG. 7. The MR head produced an output voltage of about
2.8 mV.
For comparison purposes, an MR head was fabricated as above except
that the magnetic multilayer film was replaced by a Permalloy film.
A measuring current of 15 mA was passed across the MR head while an
external magnetic field varying between -20 Oe and +20 Oe at 50 Hz
was applied. The MR head produced an output voltage of about 0.8
mV. This means that the MR head having the present invention
applied thereto produced a 3.5 times greater output than the
conventional MR head.
Example 4
Like the MR heads fabricated in Example 3, yoke type MR heads of
the structure shown in FIG. 4 were fabricated using a magnetic
multilayer film within the scope of the invention and a Permalloy
film in the magneto-sensitive section. On output measurement, the
yoke type MR head using the inventive magnetic multilayer film
produced a 1.9 times greater output than the yoke type MR head
using Permalloy.
Example 5
Like the MR heads fabricated in Example 3, flux guide type MR heads
of the structure shown in FIG. 5 were fabricated using a magnetic
multilayer film within the scope of the invention and a Permalloy
film in the magnetosensitive section. On output measurement, the
flux guide type MR head using the inventive magnetic multilayer
film produced a 2.8 times greater output than the flux guide type
MR head using Permalloy.
Japanese Patent Application No. 227347/1994 is incorporated
hereinby reference.
Although some preferred embodiments have been described, many
modifications and variations may be made thereto in the light of
the above teachings. It is therefore to be understood that within
the scope of the appended claims, the invention may be practiced
otherwise than as specifically described.
* * * * *